An alternator is a machine that generates an alternating-current (“AC”) electrical signal from mechanical energy. The alternator may receive the mechanical energy in the form of a spinning shaft known as a rotor. The rotor may create a magnetic field that causes current to flow in a stationary portion of the alternator known as a stator. There are two main ways for a rotor to create a magnetic field. First, the rotor may contain a permanent magnet that creates a magnetic field that rotates with the rotor. Alternators that use a permanent magnet may be known as magnetos. Second, the rotor may create a magnetic field by having electric current pass through windings inside the rotor.
An alternator may contain a brush that provides an excitation current to the rotor to generate an electric current through the windings inside the rotor. The brush may have a brush holder that pushes the brush against the rotor. If the brush contacts the rotor, the brush may conduct an electric current with the rotor. If there is a gap between the brush and the rotor, a brushfire may occur as the potential difference between the brush and the motor creates an electric arc across the gap.
Similar configurations may exist in other power systems such as electric motors and electric generators. An electric generator may operate on similar principles as an alternator, converting mechanical energy to electrical energy. An electric motor may convert electric energy to mechanical energy. Generators and electric motors may include rotating shafts and brushes that conduct electric current to the rotating shafts.